Temperature control devices with follower controls, operating with a drive motor and a feedback potentiometer are known, for example, German Utility Model 75 40 562.
Electropneumatic controls are also known, with two individually cycled solenoid valves and with pulse controllers connected in series, for example, German Offenlegungsschrift 2 126 599.
Follower controls for drive motors require a great many parts, some of which move rapidly. The service life of small electric motors poses difficulties, since the motors must operate with minimal temperature deviations as well as high rotational speeds. These drive motors must either withstand the total blocking moment, or limit switches must be provided to ensure reliable shutoff at the end positions. Furthermore, reliable starting at low temperatures cannot be ensured without an additional expenditure for special lubrication and for the bearings of rapidly moving parts. Since signals for clockwise and counterclockwise operation must be generated and amplified, the cost of electronic components is relatively high.
In known devices which are controllable by air, a controller must assume the heating and cooling functions when the cooled or considerably heated vehicle is started, and then must compensate for different outside temperatures. After the blocking state is reached, the intervention of the control is minimized. It is therefore tedious and costly to chop the analog signal at a relatively high frequency, merely to make minor corrections. Moreover, two solenoid valves are required for this purpose, which must be kept constantly in pulsed operation, and therefore are subjected to severe wear. Control systems with hot-wire transducers and quasi-constant controls with two solenoid valves are known. The hot-wire transducers have relatively long tensioned wires, which are traversed by different electric currents. This results in changes in length, which control a vacuum valve. The relatively large structural lengths which are required to accept the stretched wires as well as the long-term drift induced thereby and the exact adjustment which is necessary, are disadvantageous.
It is also known to provide a locking mechanism for a pneumatic control by means of a solenoid coil and an air nozzle. The blast is turned on or off as a function of a thermostat, through a relay. In the shutting-off process, an air nozzle is simultaneously closed, so that the actual temperature controller, operating with a bimetallic strip, is shut off, as shown in U.S. Pat. No. 3,460,754.